1. Field of the Invention
The invention relates generally to magnetic random access memory (MRAM). Specifically, the invention relates to a high-density memory architecture comprising a vertical stack of magnetic storage elements.
2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
MRAM devices typically consist of a planar arrangement of memory cells with two magnetic layers separated by a tunnel junction. One of the magnetic layers is a fixed reference layer while the other layer is a storage layer having a magnetic polarization that is altered for storage. The storage layer can be oriented along one of two directions along a magnetic uni-axial anisotropy axis approximately parallel or anti-parallel to the magnetization of the reference layer.
A memory write to a memory cell aligns the storage layer in either the parallel or the anti-parallel position with respect to the reference layer. A memory read determines the resistance of the memory cell being read and determines the alignment of the storage layer based on the resistance of the memory cell. Then the “value” of the memory cell is known.
One problem with the prior art is that it is difficult to manufacture MRAM cells and they require a significant amount of space, therefore yielding a low MRAM density. Furthermore, memory write requires a narrow distribution of switching fields in order to avoid writing of half selected bits or writing adjacent bits due to crosstalk. Memory read is usually performed by comparing the resistance of the cell being read to a reference cell, again requiring a relatively tight tolerances of cell resistance values across the memory chip. MRAM are therefore difficult to manufacture and have low density.
What is needed is high-density MRAM that is easy to manufacture and provides good selectivity of memory cells. The invention should reduce the area required by memory, be easily manufactured by lowering the margin requirements for memory resistance, provide improved selectivity of memory cells, and be scalable.